MRI is an apparatus detecting NMR signals (echo signal) produced from the object by applying high frequency magnetic field to the object placed in the static magnetic field, and signal processing and imaging these signals. And high uniformity of the static magnetic field is demanded in order to get image of high quality. Especially in fast imaging method, such as single-shot type or multi-shot type of an echo planar imaging (EPI) method, echo signal to be necessary for reconstructing one image is acquired with only once or several times of nuclear spin excitation. Therefore high static magnetic field uniformity is demanded, because image quality acquired is influenced by uniformity of the static magnetic field.
In an MRI apparatus, static magnetic field is formed by static magnetic field generation means such as permanent magnet or super conducting magnet, and is maintained on high uniformity, but the uniformity varies according to the susceptibility of the object placed in the measurement space. It is proposed a method for removing the influence caused by the non-uniformity of the static magnetic field to the picture by signal processing. In this method, at a certain time before measurement for the formation of the image of the object, nuclear spin in imaging part of the object is excited by applying slice selective gradient magnetic field and high frequency magnetic field to the object. Then data for non-uniformity static magnetic field correction is previously acquired in the condition without application of phase encode gradient magnetic field. And echo signals (imaging data) are corrected by using this correction data.
On the other hand as shown in FIG. 5, if the object 501 moves during imaging such as breath motion 504, motion artifact accompanied with this is generated. This is a problem in the image. The reason why this motion artifact is generated is that the relationship between a location of organ and quantity of encode is varied between each echo signal by the movement of the object 501. This artifact appears to be a false image as if image is flowing through the whole screen, and it is a big obstacle in a clinical diagnosis.
As a method to remove this artifact, motion correction method using navigational echo is known. (for example, MAGNETIC RESONANCE IN MEDICINE, 35:895-902, June 1996, Seong-Gi Kim et. al, Fast Interleaved Echo-Planar Imaging with Navigator: High Resolution Anatomic and Functional Images at 4 Tesla).
By the way, in case of continuously imaging a region close to breast such as heart or lung field, if said motion correction method is performed together with said static magnetic field non-uniformity correction method, there is a case that static magnetic field non-uniformity correction method does not work effectively. That is, by the influence of the motion accompanied with breath motion of the object, static magnetic field non-uniformity is varied, too. However, data for static magnetic field non-uniformity correction previously measured does not include the change of static magnetic field non-uniformity by the motion of the object. So it is impossible to correct the measured echo signal accurately, even if static magnetic field non-uniformity correction is performed.
Thus the object of the present invention is to improve the correction accuracy in case of removing the influence of static magnetic field non-uniformity from image using correction data.
In addition, other object of the present invention is to provide an MRI apparatus, which can acquire high quality image by combining effectively the motion correction method and static magnetic field correction method.